In the 3GPP LTE, downlink user-data transmission is carried out on the Downlink Shared Channel (DL-SCH) transport channel. In LTE, the time dimension is divided into radio frames of length 10 ms, where each radio frame consists of 10 subframes, each of length 1 ms corresponding to 14 OFDM (orthogonal frequency-division multiplexing) symbols. Each subframe consists of two slots, each of length 0.5 ms or seven OFDM symbols. Note that, in case of Time Division Duplex (TDD), only a subset of the subframes of one frame is available for downlink transmission. On the other hand, in case of Frequency Division Duplex (FDD), all subframes on a downlink carrier are available for downlink transmission.
In LTE, the overall time/frequency-domain physical resource is divided into resource blocks, where each resource block consists of twelve OFDM subcarriers during one slot. DL-SCH transmission to a UE is carried out using a set of such resource blocks during one subframe. Layer 1/Layer 2 (L1/L2) control signaling, also known as the Physical Downlink Control Channel (PDCCH), is transmitted at the beginning of each subframe. The L1/L2 control channel is typically used to inform a UE about various items. For example, the L1/L2 control channel may identify whether the DL-SCH carries data to the UE in the given subframe. More specifically, the L1/L2 control channel then includes the RNTI (Radio Network Temporary Identifier) associated with the UE for which the DL-SCH carries data in the given subframe. The L1/L2 control channel then also identifies the physical resource, more specifically the specific set of resource blocks that is used for the DL-SCH transmission to the specific UE in the given subframe. Moreover, the L1/L2 control channel then identifies the transport format (e.g. the modulation scheme and coding rate) used for DL-SCH transmission to the specific UE in the given subframe. Separate DL-SCH transmissions, using different physical resources (different resource blocks), can be carried out to different UEs during the same subframe. In this case there are multiple L1/L2 control channels, one for each UE that is to receive DL-SCH transmission in the given subframe.
In addition to user data, system information is also transmitted on the downlink within each cell. The system information may, e.g., include: public Land Mobile Network (PLMN) identity/identities, identifying the operator(s) to which the cell “belongs”; Neighbor-cell list, i.e. a list of the cells that are neighbors to the current cell; and different parameters used by the user terminal when accessing the system, e.g. random-access parameters and cell-access restrictions. The system information can be divided into two parts, one part being fixed and the other part being dynamic. The fixed part of the system information is transmitted on a pre-determined physical resource, i.e. a specific set of OFDM subcarriers during a specific time interval, using a pre-determined transport format. There is thus no flexibility in the amount of information in the fixed part of the system information. There is also no flexibility in the transmission structure (the physical resource and the transport format) used for the fixed part of the system information. In LTE, the fixed part of the system information is transmitted using the BCH (broadcast control channel) transport channel. Furthermore, for LTE it is currently assumed that the BCH is transmitted in the six centre resource blocks in subframe #0 of each frame.
The dynamic part of the system information is assumed to be transmitted using the DL-SCH, or at least a DL-SCH-like transport channel, similar to normal data transmission as described above. New UEs continuously “enter” the cell, either entering from a neighbor cell, due to power-on, or upon return from out-out-service, and the UEs must quickly acquire the system information. Thus the system information (both the fixed part on the BCH and the dynamic part on the DL-SCH or a DL-SCH-like channel) should be repeated regularly.
As an example, in LTE the fixed part of the system information (transmitted using the BCH) is assumed to be repeated every 40 ms. Also the dynamic part of the system information should be repeated more or less regularly. However, different portions of the dynamic part of the system information are more or less time critical, in the sense of how quickly the UE must acquire it, and thus need to be repeated more or less often. This can be described so that the dynamic part of the system information is divided into different so-called scheduling units, also referred to as System Information Messages. In general, information corresponding to scheduling unit number n should be repeated more often than information corresponding to scheduling unit number n+1. As an example, scheduling unit #1 (SU-1) may be repeated (approximately) once every 80 ms, scheduling unit #2 (SU-2) may be repeated (approximately) once every 160 ms, scheduling unit #3 (SU-3) may be repeated (approximately) once every 320 ms, etc.